In a typical electrorheological (ER) fluid valve, two closely spaced electrodes establish an electrical field between them to stop the flow of the ER fluid through the valve as the ER fluid becomes highly viscous in the presence of the electrical field. Removing the electrical field between the two electrodes allows the ER fluid to flow again through the valve. Cylinder type and plate type ER valves are known. See Han, Y-M; Sung, K-G; Shohn, J. W.; and Choi, S-B, “Performance Comparison of Electrorheological Valves with Two Different Geometric Configurations: Cylinder and Plate”, J. Mechanical Engineering Science, Proc. IMechE Vol. 223 Part C: pages 573-581 incorporated herein by this reference. See also Zuan, Michael, “Design of Cylinder Drives Based on Electrorheological Fluids,” International Journal of Fluid Power 7 (2006) No. 1, pp 7-13.
One advantage of using an ER valve as opposed to a traditional hydraulic valve lies in the simplicity of the valve. Two electrodes with a gap between them and the application of a voltage across the electrodes is required as opposed to moving parts associated with servo valves. An ER valve can also be made very small. Additionally, the ER effect can be made to occur quickly, within ten milliseconds resulting in fast response times and high frequency operation.
A typical ER fluid valve can be either fully open or fully closed. In order to control the flow rate through an ER valve, pulse width modulation of the electrical field in the valve can be used or variable voltages can be applied to the electrodes. A variable voltage can be used, but can be difficult to generate, especially in small-scale systems.
Traditional servo valves can increase the weight of a system, are expensive, and can include high precision components. On the other hand, controlling the flow rate of fluid through an ER valve can be challenging.